1. Field
The example embodiments in general are related to the field of Food Chemistry; that is, to the study of processes and interactions existing among biological components when foods are manipulated, as considered from a chemical point of view. More particularly, the example embodiments relate to devices and methods for testing water activity of foods, by means of which the stability and useful life of a food can be predicted.
2. Related Art
Water is an essential component of many foods. It can be found at intracellular media or as an extra-cellular component of vegetables and of products of animal origin. Its function must be understood as the one that favors dispersion of different substances, as well as that of a solvent of a great variety of chemical products. The study of water in foods is necessary, because the understanding of its properties and concentration allows, for example, the control of decay chemistry and microbiological activity in foods.
Also, the elimination (drying) or freezing of water are essential for some food conservation methods.
The presence of water in some foods is sometimes understood as a determinant part of their texture, particularly the one named water activity; therefore its analytical measurement in foods is considered as of great importance.
Water activity is defined as the ratio existing between water vapor pressure of a certain food, and the pure water vapor pressure at the same temperature. Generally it is represented by Aw (from “activity of water”). Water activity is a parameter closely related to the moisture of the food, which allows the determination of its conservation and microbial propagation properties.
A more formal definition of water activity, represented by Aw, is:
Aw=p/po, where: p is vapor pressure of water in the substance, and po is vapor pressure of pure water, at the same temperature.
In view of the importance that water activity has attained as a criterion for evaluation and control of food safety and quality, a wide diversity of instruments and methods to measure this activity are used; however these devices and instruments are generally complex in their assembly and operation because they use highly sophisticated technology, comprising high-precision sensors, sample retention chambers that are highly sensitive to environmental pressure and temperature conditions, or chemical compounds that need complex manipulation. This result in a limited application of these devices and methods, because they require accurate operating conditions, as well as the attention of specialized personnel; their sophistication level renders them among high-cost devices, hardly accessible to food producers when measurement of water activity of foods produced by them is to be done rapidly, effectively and with a minimum testing cost.
In this sense, Campbell1 describes an apparatus and a method for measuring activity and condensation temperature of water and temperature; it comprises one compartment where an air sample is measured, one air circulation device, installed inside the compartment, one sensor of sample temperature, one sensor of relative humidity, and one deflector that allows the direct influence of air circulating inside the compartment on the sensor of relative humidity. Another embodiment of this invention comprises the assembly of an air circulation device within the compartment, to force the air inside the compartment so as to reduce the resistance of the limit layer. Still another embodiment of this invention consists in using a temperature control device for controlling inner surface temperatures inside the compartment. Still another embodiment of this invention comprises one sensor for relative humidity of the product, where one sensor of condensation temperature, that can be a resistive or capacitive sensor, is used jointly with a heater and/or a fan for the purpose of shortening the time required to reach equilibrium, and improving accuracy of condensation temperature measurements.
Friedrich2 describes a device for measuring water activity of foods not containing water, which is used for measuring activity of water in the substances, especially the food substances such as meat and meat products not containing water. This device comprises one lower section and one upper section, detachably connected to the lower section. The lower section contains one or more measuring instruments, including a container for containing the food sample to be analyzed. On the container an instrument support is mounted, to hold the adequate instrument with its respective indicator and a visible scale, and the container has an opening in the upper section. Upper and lower section consists, preferably, of a synthetic foam material, such as hard polystyrol foam.
Finally, and this does not mean that no more documents on status of the art exist reflecting sophisticated inventions developed by men for measuring water activity, but only to clarify which are the antecedents of the invention matter of this patent application, Sharpe3 describes one instrument for measuring relative humidity or the level of water activity of a food sample, within set limits, that obtains a reading in matter of seconds; the instrument comprises a rough or porous exposition surface, on which there is a pattern of substances named indicators, and each of them are dissolved at a known relative humidity. These indicator substances are applied in such quantities that they are invisible, or nearly invisible, when dry, but they become nearly of entirely visible again when contacting a humid atmosphere, depending on the relative humidity of the atmosphere, referred to as the range of relative humidity covered by indicators that are dissolved. If indicators are applied in the form of decimal fractions corresponding to their relative humidity, the instrument gives a digital reading of relative humidity, directly and with no electronics. In an embodiment, the exposition surface is made of crystal treated with dichlorodimethylsilane once indicators are applied. In other embodiments, exposition surface is a fine porous film made of polymer. In other embodiments, the exhibition is included in a low dish that can be applied to the surface of a food. In a final embodiment, dissolving indicators are mixed with a dye, which is diffused into a porous detachable layer, thus providing a permanent record of moisture or water activity.
Considering the above background, the development of sample and effective methods and devices for determining water activity of foods is essential.